1. Field of the Invention
The present invention relates to a discharge lamp starter for controlling lighting of a discharge lamp used for the headlights of vehicles, for example.
2. Description of Related Art
It is sometimes necessary for vehicles such as automobiles and motorcycles to travel with lighting the headlights even during the daytime in order to improve visibility from oncoming vehicles to maintain traffic safety. To meet such a need, a headlight control unit has been developed for carrying out the daytime running light (abbreviated to “DRL” from now on) in addition to the normal nighttime running light (called “normal light” from now on) (see the following Relevant Reference 1, for example).
The headlight control unit turns on the headlights at a specified luminance required for the normal light during the night. In contrast, during the daytime, it varies the luminance of the headlights within the luminance less than that required for the normal light in response to the brightness outside the vehicle detected by a photodetector.
With the configuration, the conventional headlight control unit can prevent the drivers of oncoming vehicles and pedestrians from being dazzled by the headlights during the daytime running light in a rather light environment. In addition, since it can prevent the excess luminance of the headlights, it can curb the waste of power. Furthermore, since it enables the headlights to provide enough luminance to a rather dark environment, it can prevent the reduction in the visibility of the drivers of the oncoming vehicles and pedestrians.
Relevant Reference 1: Japanese patent application laid-open No. 2001-347880.
Recently, high-intensity discharge lamps (called HID lamps from now on) such as metal halide bulbs, high-pressure sodium bulbs and mercury bulbs have been applied to the headlights of vehicles because they have advantages of large luminous flux, high lamp efficiency and long life.
The HID lamps, however, require large power at the turn on because of the necessity to increase the temperature of the electrodes. The electrodes of the HID lamps are impaired because they are scattered by the large current at the start. Because of an increasing number of times of turn on and off of the HID lamps during the DRL control, the electrodes of the HID lamps are impaired more severely in the DRL control than in the normal light control. As a result, the life of the HID lamps is reduced so that users must replace the HID lamps at short intervals, thereby imposing a heavy load on the users.
In addition, when switched from the normal light to the DRL, the HID lamps can reduce the luminance abruptly and impair the visibility, which is unfavorable for the drivers.